A standard hypodermic syringe comprises a cylindrically tubular body having a front end closed by a plug formed with a central passage extending along the axis of the body and a rear end provided with a piston longitudinally axially displaceable in the body. A quantity of liquid to be injected is held in the body between the piston and the plug and a needle is fitted to the passage at its front end so that forward displacement of the piston by a plunger forces the liquid out of the body through the needle.
A syringe for one-time use is frequently supplied already containing a lyophilized medicament, to which some solvent, for instance sterile water, is added to make the dried medicament injectable. To prepare the syringe for use a needle is mounted on the front end of the body and the liquid is drawn into it.
It is also possible as described in U.S. Pat. No. 4,874,381 for there to be two pistons, namely a front piston that subdivides the interior of the body into a rear compartment holding the dry medicament and a front compartment holding a solvent, typically water, for it. In this case the body is formed with a bypass that permits mixing of the medicament and the liquid on axial displacement of the pistons. A rear piston is advanced to eject the contents.
In both systems the syringe is initially prepared by filling into the body a quantity of the dissolved medicament and then lyophilizing it and driving off the vaporized solvent, which escapes through the open front end of the cylinder. While maintaining sterility the front end is plugged, and the thus prepared syringe is then fitted with the necessary tip cap or the like, and is bagged.
Such an arrangement is not readily susceptible to mass production, as it is necessary to hold down costs of this throwaway item. In fact production is fairly difficult in view of the problems associated with plugging the cylinder end while maintaining the syringe and its environs sterile.